An allergic reaction is an undesirable immunoreaction resulting from the response of an antibody or a sensitized cell to an antigen. An antigen causing an allergic reaction is specifically called an allergen. Allergens include a wide range of substances, such as pollen, mites, animals' epidermis, insects, foods, drugs, and chemicals. An allergic reaction is generally characterized by a two-phase reaction comprising an immediate reaction to an allergen and a subsequent delayed reaction. In the early stage of an allergic reaction, an allergen-specific IgE antibody binds to the surface of basophils in the peripheral blood and mast cells in tissues. When an allergen enters the body, the IgE antibodies on the surface of basophils or mast cells cross react with the allergen. As a result, inflammatory mediators including histamine, prostaglandin, and leukotriene are released. In response to these inflammatory mediators, locally accumulated lymphocytes, monocytes, basophils, and eosinophils are activated and release mediators causing tissue damage and other various responses in tissues, thereby initiating a delayed reaction.
It is well known that an allergic reaction is controlled by cytokines. Cytokines are involved in not only the control of IgE production but also the activation and differentiation of effector cells. This is supported by the observation that the level of an allergen-specific IgE in the blood is constant even if clinical symptoms of an allergy patient have been alleviated by hyposensitization.
Hyposensitization, a method for treating allergic diseases, comprises administering a small amount of antigen (for example, an antigen extracted from cryptomeria pollen or mites) to an allergy patient, and increasing the dosage gradually. The success of hyposensitization is attributed to the decreased response of allergen-specific T cells. Presumedly, hyposensitization causes T-cell tolerance (T cell anergy), and, as a result, cytokine, which is important for developing an allergic cascade, is not produced. Studies on allergies have focused on the allergen-specific immunoreaction in the early stage, especially on the mechanisms for controlling T-cell response to allergy. An allergic response to an exogenous antigen including an allergen is initiated depending on antigen-presenting cells in the immune system. Antigen-presenting cells, including B cells, macrophages, and dendritic cells, incorporate exogenous antigens, fragment the exogenous antigens into antigen peptides (T-cell epitope peptides), and express the fragmented antigens on the cell surface together with MHC class II (HLA class II for a human) to present an antigen to antigen-specific CD4 positive helper T cells (Th cells).
HLA class II molecules (DR, DQ, and DP) are cell surface antigens composed of α and β chains. The α chain of the DR molecule is encoded by the HLA-DRA gene; the β chain of the DR molecule is encoded by HLA-DRB1, HLA-DRB3, HLA-DRB4, or HLA-DRB5 genes. The α and β chains of the DQ molecule are encoded by HLA-DQA1 and HLA-DQB1 genes, respectively, while α and β chains of the DP molecule are encoded by HLA-DPA1 and HLA-DPB1 genes, respectively. Except for HLA-DRA, each gene comprises numerous alleles. Pockets accommodating antigen peptides composed of α and β chains show high polymorphism, and their structures differ slightly from each other. As a result, kinds of antigen peptides binding to pockets and presented to T cells are limited by their structure. This presumably produces differences in individual immunoreactions.
Th cells that receive the antigenic information restricted by HLA class II molecules through T-cell receptors (TCR) are activated and secrete various cytokines to proliferate by themselves and differentiate B cells into plasma cells, thereby inducing antibody production. At this time, the second signal (costimulatory signal), which is mediated by molecules other than TCR, is necessary to activate T cells. In contrast, without this signal, immunological tolerance of Th cells to an antigen is induced (June, C. et al.: Immunol Today, 15: 321, 1994).
The decrease of T-cell response to an allergen is related to the success of hyposensitization. For example, the T-cell response in vitro to ambrosia allergen “Amb a 1” in a patient suffering from an ambrosia allergy who had undergone effective hyposensitization for ten years was dramatically decreased compared to an untreated patient. Similarly, in a patient allergic to feline epidermis antigen “Fel d 1,” T-cell response specific to Fel d 1 was obviously decreased, as hyposensitization showed effects. This decrease corresponded to the decrease of sensitivity in the skin test. Furthermore, IgG and IgE antibodies specific to Fel d 1 remained at a constant level during the treatment. These results indicated that a therapeutic agent for an allergy directly targeting antigen-specific T cells could be prepared.
Development of biochemical separation and analysis techniques has enabled purification of various allergens. In particular, more than 100 kinds of allergen genes have been cloned and their primary structures have been determined in the last several years using techniques in molecular biology and genetic engineering. T-cell epitope sites were also identified in some of those allergens.
Peptide-based immunotherapeutic compositions using peptides including T-cell epitopes of allergens have been disclosed (International patent application published in Japan Nos. Hei 7-502890, Hei 8-502163, and Hei 8-507436). When some parts of a T-cell epitope of feline allergen Fel d 1 molecule, but not all of it, were subcutaneously administered to a mouse, antigen-specific T-cell tolerance was reportedly induced against the challenge of whole Fel d 1 (Briner, T. J. et al.: Proc. Natl. Acad. Sci. USA, 90: 7608-7612, 1994). However, whether a major T-cell epitope is effective enough to decrease T-cell response to the challenge of a whole allergen and whether this is can alleviate clinical symptoms have not been confirmed by clinical experiments in humans.
Reportedly, about 3 to 16 T-cell epitope sites exist in an allergen molecule, of which about 1 to 7 sites are recognized by a patient. When the HLA class II type differs in each patent, T-cell epitope sites recognized by each patient also differ. When the HLA class II type is the same, the same T-cell epitope sites are recognized. Thus, the above-described peptide-based immunotherapy using a peptide containing only one major epitope of an allergen molecule recognized in a particular patient population cannot be effective for all patients.